The present invention relates to a DC/DC converter with a half-bridge configuration.
FIG. 5 shows a conventional example. A series circuit of a MOSFET 1 (Metal Oxide Semiconductor Field Effect Transistor) and a MOSFET 2, and a series circuit of capacitors 5 and 6 are connected parallel between positive and negative electrode sides of a DC power supply 7. Also, a reactor 14 and a primary winding of a transformer 11 connected in series are located between a connection point of the MOSFETs and a connection point of the corresponding capacitors. Further, snubber capacitors 3 and 4 are connected parallel to the corresponding MOSFETs. A transformer 11 has two windings on its secondary side, and has a rectifying and smoothing circuit composed of diodes 9 and 15 and a capacitor 10. Moreover, an output voltage detection circuit 12 and a control circuit 13 provide feedback control to maintain a smoothed DC output voltage at a constant value.
In such a circuit, the MOSFETs 1 and 2 are alternately turned on and off with a certain duration of a short-circuit preventing time Td between the turn-on and turn-off to execute switching, so that the switch elements have equal control signal pulse widths. This controls the switching frequency to produce constant DC output voltage.
The operation of the DC/DC converter shown in FIG. 5 will be described with reference to FIG. 6.
When one of the MOSFETs is turned off, the snubber capacitor connected parallel to the MOSFET turned off is charged to a DC power supply voltage Ed by an excitation current flowing through transformer 11, while the snubber capacitor connected parallel to the other MOSFET is discharged to zero voltage. The voltage of the MOSFET that is turned off increases gradually due to charging of the snubber capacitor, achieving zero voltage switching and small turn-off loss. When the voltage across the snubber capacitor becomes zero, a body diode of the other MOSFET becomes electrically conductive. At this time, this MOSFET is turned on, resulting in zero voltage switching and no turn-on loss. Further, the capacitors 5 and 6 and reactor 14 constitute a series resonant circuit, so that a resonant current from the resonant circuit flows through the diodes 9 and 15. Accordingly, the each of the MOSFETs is switched after a current flowing through each of the diodes becomes zero, so that the diodes are subjected to soft recovery, with almost no resulting reverse-recovery loss.
As described above, the MOSFETs have low switching loss while the diodes have low reverse-recovery loss, resulting in improved efficiency for the DC/DC converter.
However, since a DC output voltage is controlled by using a switching frequency, when the circuit carries a light load, the switching frequency rises, increasing switching loss and circuit loss associated with charging or discharging of the snubber capacitors. As a result, efficiency is reduced during the period in which the circuit carries a light load. This will be explained below.
FIG. 7 is a modeled circuit of FIG. 5. In FIG. 7, reference C denotes a parallel capacity of the capacitors 5 and 6, while reference L denotes a series inductance of an inductance of the reactor 14 and a leakage inductance of the transformer 11. Reference Z denotes an equivalent impedance of a load connected to an output terminal. A DC output voltage Vo is shown by Equation (1).
Vo=Vs/(1+|xcfx89Lxe2x88x921/xcfx89C|2/|Z|2)xc2xdxe2x80x83xe2x80x83(1)
Equation (1) indicates that as the load decreases, i.e. Z increases, xcfx89 must be increased in order to keep the output voltage constant, resulting in increased switching frequency.
A potential at a source terminal of the MOSFET connected to a positive side of a DC power supply differs from that at a source terminal of the MOSFET connected to a negative side of the DC power supply. Thus, a signal for driving the positive-side MOSFET must be insulated by a pulse transformer, or a high-voltage-resistant IC with a level shift function is required, increasing the number of required parts and costs.
It is thus an object of the present invention to provide a DC/DC converter which minimizes an increase in switching frequency while the circuit carries a lighter load, thereby minimizing the need for expensive parts and reducing costs.
Other objects and advantages of the invention will be apparent from the following description of the invention.
In the first aspect of the invention, a DC/DE converter for a DC power supply is formed of a series circuit including a first switch element and a second switch element, which is connected between a positive electrode side and a negative electrode side of the DC power supply; a transformer having a primary winding connected at one side to a common connection point of the first and second switch elements, and a secondary winding with outputs; at least one capacitor connected between a line extending from the primary winding to the common connection point of the first and second switch elements and one of the positive and negative electrode sides of the DC power supply; a diode connected in series to the secondary winding; and a smoothing capacitor situated across the secondary winding outside the diode. A control circuit is connected to the first and second switch elements for switching ON and OFF the first and second switch elements alternately. The control circuit varies an ON period of the first switch element and switches ON the second switch element for a predetermined period.
The DC/DC converter of the second aspect is based on the first aspect, wherein the control circuit includes a first control section connected to the first switch element for controlling the same, and a second control section connected to the second switch element for controlling the same. The transformer further includes a first auxiliary winding and a second auxiliary winding. Also, the DC/DC converter further includes a first rectifying and smoothing circuit connected parallel to the first auxiliary winding for feeding electric power to the first control section, and a second rectifying and smoothing circuit connected parallel to the second auxiliary winding for feeding electric power to the second control section.
In a DC/DC converter of the third aspect, the DC/DC converter based on the second aspect further includes a first voltage switching-timing detection circuit connected parallel to the first auxiliary winding for generating a first timing signal for switching ON the first switch element when the voltage across the first auxiliary winding changes from a positive voltage to a negative voltage, and a second voltage switching-timing detection circuit connected parallel to the second auxiliary winding for generating a second timing signal for switching ON the second switch element when the voltage across the second auxiliary winding changes from a negative voltage to a positive voltage.
In the fourth aspect of the invention, the DC/DC converter in the third aspect further includes a voltage detection circuit connected to the smoothing capacitor for detecting DC output voltage across the smoothing capacitor. The first control section switches ON the first switch element in response to a first timing signal and varies the ON period of the first switch element to keep a value of the detected DC output voltage constant.
The DC/DC converter of the fifth aspect is based on the third aspect, wherein the second control section switches ON the second switch element in response to a second timing signal and keeps the second switch element switching ON for the predetermined period. The DC/DC converter operates properly even if the circuit carries a light load.